The use of servo flaps for pitch positioning the blades of a helicopter main rotor is well known. For example, U.S. Pat. No. 3,095,931 shows a helicopter having a single four-bladed main rotor having a pitch positioning servo flap associated with each of the rotor blades; and U.S. Pat. No. 2,936,836 shows a helicopter having two two-bladed intermeshing main rotors with each blade of each rotor having a pitch positioning servo flap. In the past such servo flaps have customarily been located rearwardly of the trailing edges of their associated rotor blades. Such trailing edge flaps in combination with their associated rotor blades are essentially stable. That is, when the servo flap is moved about its own pitch axis the blade and flap move toward a balanced condition at which the pitch changing force applied to the blade by the servo flap is opposed by the aerodynamic pitch changing force applied to the blade.
Trailing edge servo flaps do, however, have the disadvantage that they work, in general, in opposition to the aerodynamic load requirements of their associated rotor blades. For example, when a rotor blade is required to be generating lift at high angles of attack, a trailing edge servo flap must produce a load in opposition to lift in order to properly position the blade. This negative lift from the servo flap decreases the overall lifting efficiency of the rotor system. If servo flaps are placed at the leading edges of the rotor blades rather than at the trailing edges they work, in general, in agreement with the aerodynamic load requirements of the rotor blades so that in comparison to a rotor having trailing edge servo flaps a rotor with leading edge servo flaps offers the possibility of substantially improving the overall efficiency of the rotor.
The possibility of increasing rotor efficiency through the use of leading edge servo flaps, or a combination of leading edge flaps and trailing edge flaps, has been recognized for some time, but past attempts at utilizing leading edge flaps have basically been unsuccessful, due largely to a rotor blade with a leading edge flap being a relatively unstable system. For example, if the rotor blade is to be moved to an increased pitch angle the leading edge servo flap must first be moved to an increased angle about its own pitch axis. Then as the rotor blade starts to move to the new pitch angle the lift on the servo flap is still further increased and will quickly drive the rotor blade beyond the desired pitch position unless the servo flap is moved soon enough in a compensating way. The control requirements for leading edge servo flaps are therefore complex and difficult to define and are of such nature as to be difficult to transmit to the servo flaps through swash plates and other customarily mechanical helicopter rotor blade control mechanisms.
U.S. Pat. No. 4,899,641 shows an electro-hydraulic helicopter control system wherein blade pitch control signals are transmitted to the blades as hydraulic signals transmitted through an hydraulic "slip-ring" or stationary-to-rotating interface essentially replacing the swash plate of a mechanical control system and overcoming the control limitations of a swash plate. In the system of said patent, rotor blade pitch control signals are computed from various real-time input signals, including pilot command signals and operating parameter signals and are converted from electrical signals output by the computer to hydraulic signals supplied to hydraulic actuators rotating with the rotor which directly control the pitch positions of the rotor blades. Associated with each blade pitch hydraulic actuator is a simulator, carried by the stationary structure of the helicopter, having identical characteristics to the real actuator and which simulator supplies a feedback signal defining the actual pitch position of the blade and used as one of the input signals to the computer which calculates the blade pitch position signals.
The object of the invention is, therefore, to provide a control system for a helicopter main rotor using servo flaps for pitch positioning the rotor blades and which increases the efficiency of the rotor by using leading edge servo flaps in place of the customary trailing edge servo flaps and which overcomes the stability problems previously associated with leading edge servo flaps.
In keeping with the aforegoing object, a more specific object of the invention is to provide a rotor blade pitch control system using leading edge servo flaps in combination with a control signal generating and transmitting system capable of generating the complex control signals required by the servo flaps and of transmitting them to rotor pitch actuators rotating with the blades.
Further objects and advantages of the invention will be apparent from the following detailed description of a preferred embodiment of the invention and from the accompanying drawings and claims.